This invention relates to a vehicle multiplex communication system comprising a plurality of nodes on board of a vehicle connected to a bus and circulating the transmission right to the nodes in a predetermined order for the node getting the transmission right to send a predetermined number of data pieces to the bus.
In a vehicle such as an automobile, a plurality of electronic control nodes (electronic control units) are connected to a common bus and data is transferred between the nodes. In this case, token passing for circulating the transmission right to the nodes in a predetermined order for the node getting the transmission right to send a predetermined number of data pieces to the bus is known.
An art of an on-board electronic control system adopting this type of token passing is described, for example, in Unexamined Japanese Patent Application Publication No. Sho 62-51331, wherein a plurality of electronic control units are connected to one signal line and send a predetermined number of data pieces to the signal line in a predetermined order. Each electronic control unit is configured as shown in FIG. 12.
As shown in FIG. 13, after a lapse of predetermined data transfer stop time T0, an electronic control unit 110 shown in FIG. 13 outputs a predetermined number of data pieces (0-3) via a transistor 133 from an output terminal T to a signal line 120. Upon completion of the transfer of the data pieces as many as the predetermined number, likewise another electronic control unit 111 transfers a predetermined number of data pieces (0-4) to the signal line 120.
Further, as shown in FIG. 13, an electronic control unit 112 transfers a predetermined number of data pieces (0-2) to the signal line 120, then an electronic control unit 113 transfers a predetermined number of data pieces (0-5) to the signal line 120. If any of the electronic control units requires the data pieces as many as the predetermined number on the signal line 120, it receives the data.
However, each electronic control unit in the conventional on-board electronic control system transmits a predetermined number of data pieces, thus data also containing unchanged data in the data pieces as many as the predetermined number is transmitted repeatedly. Resultantly, the unchanged data occupies the bus unnecessarily, the time efficiency of the data transmission worsens, and the data transmission delay cannot be shortened.